Debris Fan Produced by Failure of Canyon-Blocking Pyroclastic Flows

Ash-rich pyroclastic flows from the cataclysmic eruption of Mount Mazama (~7700 yr. B. P.), Cascade volcanic arc, Oregon, entered and blocked the narrow, bedrock-lined canyon of the Williamson River approximately 35 to 44 km from the source volcano. The blockage impounded a body of water which then released producing four stratigraphic units in the downstream debris fan. The four stratigraphic units are a boulder core comprised of locally sourced bedrock boulders and three sand-rich units including a fine-grained sand unit, a sandy pumice gravel (±basalt/hydrovolcanic tuff) unit, and a pumice pebble-bearing, crystal-rich sand unit. Hand-drilled auger holes up to ~1.6 m deep were used to obtain samples of the sand-rich units. Units were delimited using surface and down-hole observations, composition and texture, estimated density, statistical parameters of grain size, and vertical and lateral distribution of properties. Overtopping followed by rapid incision into the ash-rich pyroclastic flows progressively cleared the canyon, but a bedrock knickpoint near the head of the canyon limited the volume of debris available for transport to about 0.04 km3 to 0.08 km3. Co-deposition of bedrock boulders and lithic-rich sand was followed by rapid deposition with minimal reworking of remobilized pyroclastics. Continued draining of the impounded lake sent hyperconcentrated flows onto the debris fan depositing pumice-rich gravels that graded upward to crystal-rich sands.

Ground- and Surface-Water Interactions of a Pumice Aquifer in a Headwaters Watershed: Round Meadow, Fremont-Winema National Forest, Oregon, USA

Plinian pumice fall from the Holocene eruption of Mount Mazama in the Cascade volcanic arc is an unconfined, perched aquifer in south-central Oregon. The pumice aquifer provides near-surface groundwater storage that maintains biologically diverse wetland environments. Wetland environments reflect post-eruption disruption of the once uniform pumice blanket by fluvial and lacustrine processes operating within the template of the pre-eruption landscape. In the 8.6 km2 Round Meadow watershed the pumice aquifer interacts with a seasonally flooded meadow, fen, springs, and perennial stream. The laterally uniform, isotropic pumice aquifer is disrupted by flat-bottomed ephemeral stream valleys that drain to the seasonally flooded meadow. Surface water levels in the seasonally flooded meadow are controlled by a knickpoint developed on bedrock. The underlying aquifer is confined by a layer of glass-rich diatomaceous silt grading upward to organic-rich silt. Here, the aquifer is comprised of remnants of the pumice deposit, lag sand, and reworked pumice. The water level in the confined aquifer is maintained by recharge from the unconfined pumice aquifer following flow pathways beneath ephemeral stream valleys. The fen is developed on a down-thrown block of welded tuff and pre-eruption diatomaceous silt. Water levels in the fen are sensitive to inter-annual variations in precipitation. Low discharge, low temperature (5.0°C to 6.5°C), and low conductivity (30 to 50 μS/cm) springs appear to be fracture controlled and rising through welded tuff. Spring discharge and seepage through pumice from the welded tuff support perennial flow in the creek that also carries discharge from the seasonally flooded meadow when water levels are high enough to cross the knickpoint.

Hydrogeology of Pumice-Hosted Fens in the Winema-Fremont National Forest, Oregon, USA

Subaerial fallout from the Holocene eruption of Mount Mazama in the Oregon Cascade Range was deposited upon relatively low permeability volcanic and volcaniclastic bedrock and regolith. In the Walker Rim study area, erosion by ephemeral streams shortly after the eruption disrupted the lateral continuity of the 270 to 300 cm-thick pumice deposit. Co-evolution of the surface- and ground-water systems in a low-relief, low-slope landscape allowed diffuse groundwater discharge from the banks of the evolving stream system. Accumulation of organic material from groundwater dependent ecosystems at these sites of discharge allowed peat deposits to form on gently sloping erosion surfaces cut into the pumice deposit. Following early stream incision, fine-grained, silt-rich deposits accumulated in valleys and contributed permeability barriers to the lateral migration of water in the pumice aquifer. Fens discharge from the pumice aquifer through gently sloping surfaces patterned after the slope of the erosion surface cut into the pumice deposit and overlain by approximately 1 m of peat on the sloping surface and alluvium or iron-cemented pumice overlain by alluvium at the toe of the slope. The predominant source of groundwater is snowmelt which infiltrates the pumice deposit during the freshet. However, shallow groundwater flow also takes place along permeable pathways in bedrock units. Locally, low volume discharge takes place along faults. The snowmelt-dependent hydrologic system that supports the fens of the Walker Rim study area occurs at elevations primarily above 1585 m.